The parents of a Breton servant girl, Olga Boulanger have lodged a complaint with the police, and M has gone to Neuilly to investigate. Their daughter had died mysteriously, while working at the home of Dr. Armand Barion, and it was found that she had been four months pregnant. Dr. Barion says the father of the child was his chauffeur/valet, Martin Vignolet. The autopsy revealed that her intestine had been perforated as a result of ingesting rye spears, according to Dr. Barion, a method of murder known in Malaya and New Hebrides. The Doctor has discovered that a tramp who came regularly to the house on Mondays, and thus was called Mr. Monday, regularly brought the children cakes called religieuses, and rye spears had been found in the cakes. Olga had eaten them in place of the children., and so it was the children who had been the targets. Mr. Monday was an innocent, but at the cake shop M learned that one of Barion's neighbors, an Englishwoman, Laurence Wilfur, often ordered cakes by mistake and sent back what she had received. M learns that she was crazily in love with Barion, and had poisoned the cakes in a way that natives had killed her father when he was a Colonial officer. She spent the rest of her days in an asylum.